The object of the current study is to elucidate the functional activities and cellular and molecular mechanisms of development of CD4+ and CD8+ T-cells that mature in the mouse bone marrow microenvironment instead of the conventional thymus microenvironment. We have developed an in vitro culture system in which bone marrow cells which are stringently depleted of T-cells generate CD4+ and CD8+ T-cells within 48 hours. This marrow system recapitulates the key features of in vitro thymic culture systems in that TCR alpha and beta gene rearrangements are followed by the development of CD4+CD8+ (double positive; DP) intermediary T-cells which rapidly give rise to single positive (SP) CD4' and CD8+ cells. Positive and negative selection occurs during the transition from the DP to the SP T-cells in the marrow and thymic culture systems. The outgrowth of T-cells in the marrow system is extremely sensitive to feed-back inhibition by mature cells. The aims of the proposed study are to test the functional capacities of the marrow-derived T-cells in several in vitro and in vivo assays including secretion of cytokines by anti-CD3 stimulation, proliferative and cytolytic responses to alloantigens and ability to induce acute graft versus host disease, protect against murine CMV infection, and augment antibody responses in vivo. In addition, we will attempt to identify and compare the T-cell precursors and the stromal cells that support maturation in both the thymic and marrow culture systems. The molecular and cellular basis of the feedback inhibition by mature T-cells in the marrow system will be determined, and compared to that in the thymic system. The elucidation of the marrow pathway for T-cell development has important implications for human health and disease. If we are able to grow large numbers of functional mature T-cells ex vivo from precursors in the marrow, then this may be a source of immune reconstitution in patients with late stage AIDS whose viral infection is controlled by drugs. Such patients have marked depletion of the T-cell pool, and irreversible damage to the thymus. Thus, a non-thymic source of autologous T-cells may be of great value in this setting. In addition, elderly individuals with poor thymic function who receive bone marrow or stem cell transplants may profit from rapid immune reconstitution via this ex vivo approach also.